1. Field of the Invention
The invention disclosed herein pertains to processes used to manufacture elastomeric membranes used in vacuum pressure molding processes and, more particularly, to processes in which a breather pattern is incorporated into the surface of the elastomeric membrane.
2. Description of the Related Art
The production of various types of laminated or composite articles uses a process known as vacuum bagging or vacuum pressure molding. In the process, an impervious or non-porous envelope or membrane made of flexible material is disposed around the article to be laminated. The envelope or membrane is connected to a vacuum pump which evacuates the air inside the envelope or membrane and causes compressive pressure to be applied to the article.
In order to facilitate removal of the air inside the envelope or membrane, porous breather material is often placed between the membrane and the article. Typically, the breather material is made of polyester felt, coarse woven cotton, or fiberglass cloth.
To eliminate the use of breather material, silicone rubber membranes have been developed with patterns formed therein which enable air to escape to a vacuum port during use. One method used to manufacture silicone rubber membranes incorporating a breather pattern is to place a flat sheet of silicone rubber against a surface which will transfer its pattern to the sheet of silicone rubber when cured. This surface might be a machined metal plate, perforated metal, screen made of metal, plastic, or fiberglass, or coarse fabric. Because of the high labor and material costs associated with this method, membranes produced by this method are relatively expensive to produce.
Another method used to manufacture silicone rubber membranes incorporating a breather pattern formed therein is to interleave a sheet of coarse, closed-weave fabric treated with a release agent into a roll of uncured silicone rubber. A closed-weave fabric has very little space or no space between the threads. After the silicone rubber cures, the silicone rubber is unrolled and the sheet of fabric is removed thereby leaving an impression of the fabric design on one surface of the silicone rubber. One problem with this method is that the pattern created on the surface of the silicone rubber is shallow. As a result, the pattern is lost at high vacuum pressures and, more importantly, under autoclave conditions and does not allow air to escape.
Still another method of incorporating breathing characteristics into silicone rubber is to bond breather material to one surface of the breather material. In a vacuum pressure molding situation, this prevents the silicone rubber from stretching and conforming to the article being molded, causing high and low pressure areas on the article. Also, any resin that flows during the vacuum pressure molding process, will contaminate and fill the breather material, rendering it useless for further cures.
A cost-efficient process for high volume manufacturing of a sheet of elastomeric material with a well defined pattern formed therein which resists deformation under vacuum and autoclave conditions would be highly desirable.